Part # AN1626 datasheet

Part Manufacturer: ST Microelectronics

ST Microelectronics


Part Details:

AN1626 APPLICATION NOTE DESIGNING TWO PHASE INTERLEAVED HIGH DENSITY DC/DC CONVERTER USING L6917B by Raimondo Vai Initially developed for CPU power supply, the Multi-Phase topology is now appliable also to highdensity DC/DC converter, tipically where the voltage conversion starts from the 12V bus (output of anOFF-Line Converter) and the current need is in excess of 30A. The Multi-Phase Topology benefits ininput filter and components size reduction resulting in power density increase that allows powersupply to reduce size and increase efficiency and performances. This paper discusses the design of a high-current DC/DC converter module using L6917B a Two-Phase high performance controller. INTRODUCTIONThe The L6917B is an integrated circuit realized in BCD technology. It provides complete control logic and pro-tections for a high performance two-phase interleaved step-down DC-DC converter. It is designed in order todrive N-Channel Mosfets with a 180 deg phase shift between the two phases allowing reduction in the inputcapacitor current ripple, reducing also the size and the losses. The output voltage of the converter can be pre-cisely regulated, programming the VID pins, from 1.100V to 1.850V with 25mV binary steps, with a maximumtolerance of ±0.8% over temperature and line voltage variations. Output voltage can be further increased up tomore than 5V simply adding an external voltage divider. The device provides an average current-mode controlwith fast transient response. It includes a 300kHz oscillator externally adjustable through a resistor. Each phaseworks at the frequency fixed by the oscillator so that the resulting switching frequency at the load side results inbeing doubled. The error amplifier features a 15V/µs slew rate that permits high converter bandwidth for fasttransient performances. Current information is read across the lower mosfets RdsON or across a sense resistorplaced in series with the LS mosfet in fully differential mode. The current information corrects the PWM outputin order to equalize the average current carried by each phase. Current sharing between the two phases is thenlimited at ±10% over static and dynamic conditions unless considering the sensing element spread. The deviceprotects against over-current, with an OC threshold for each phase, entering in constant current mode. Sincethe current is read across the low side mosfets, the constant current keeps constant the bottom of the inductorscurrent triangular waveform. When an under voltage is detected the device latches and the FAULT pin is drivenhigh. The device performs also over voltage protection that disables immediately the device turning ON the low-er driver and driving high the FAULT pin. DESIGN REQUIREMENTSThe two-phase topology can be applied to several different applications ranging from CPU power supply (which Obsolete Product(s) - Obsolete Product(s) was the original application using this kind of topology) to standard high current DC-DC power supply. The ap-plication benefits of all the advantages due to the two-phase topology starting from input current and output volt-age ripple reduction up to dynamic performance enhance. This paper will present the design of the L6917B demo board in order to realize a high current DC/DC conver-sion with different output voltages: 1.8V, 2.5V, 3.3V and 5V will be considered. Output current will be in excessof 30A.Device description is introduced by first. December 2002 1/30 AN1626 APPLICATION NOTE PIN CONNECTIONS LGATE1 1 28 PGND VCCDR 2 27 LGATE2 PHASE1 3 26 PHASE2 UGATE1 4 25 UGATE2 BOOT1 5 24 BOOT2 VCC 6 23 PGOOD GND 7 L6917B 22 VID4 COMP 8 21 VID3 FB 9 20 VID2 VSEN 10 19

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